The concept of BF (Beam Forming) derives from smart antenna (SA). The basic principle of SA is that forming a directional beam via process at transmitter using the correlation of channel impulse response of half wavelength-distance antennas array to increase the signal-to-noise ratio (SNR) at receiver and extend coverage area of system. Traditional BF is usually acting on single signal stream, that is, the transmit signal is multiplied with a weight factor and then is transmitted via a plurality of antennas. Normally such gain is called as array gain. Then, the detailed meanings of BF extends with the continuous deeper research of multiple-input and multiple-output (MIMO) system, in which distance of antennas is not limited to half wavelength but may be 4 wavelength/10 wavelength, etc. In these cases, the correlation of channel matrix will weaken largely, and this non-correlation may bring diversity gain, the normal method is STBC (Space Time Block Code); this non-correlation may also bring multiplexing gain, the normal method is v-blast. In this sense, BF has the same meaning as precoding of MIMO, in other words, such BF may be seen as an implementing manner of precoding. The common BF operation manner, such as characteristic root BF, its typical meaning is that BF vector is determined via singular value decomposition (SVD) using the second order statistic information of channel (namely correlation matrix of channel). When single stream is sent, the characteristic vector corresponding to the maximum characteristic value is selected as BF vector; when multiple streams are sent, it is selected in sequence according to the order of size. For MU (Multiple Users)-MIMO, BF may be equal to SDMA (Space Division Multiple Access), wherein BF matrix or precoding matrix needs to maximize SINR signal-to-interference-plus-noise ratio (SINR) of single user, and decreases Interference among users as far as possible, and common algorithms is such as block diagonal (BD) zero forcing algorithms, etc.
In R8 of 3GPP LTE, MBMS has typically been deployed by two scenarios, that is, single-cell (SC) and Multimedia Broadcast Single Frequency Network (MBSFN) transmission. With the evolution of LTE to LTE-A (LTE-Advanced), further enhanced MBMS is required in LTE-A requirement documents namely in TR 36.814 frame according to the conclusion of last Warsaw meeting on Jun. 30-Jul. 4, 2008.
For the SC MUMS transmission, some advanced physical layer technologies, for example, HARQ, link adaptation and multi-antenna techniques, etc. may be used to improve the receiving performance of the UEs with some feedbacks over air interface from terminals (user equipment, UE) to base station (eNB). As one of the improved technologies, a “hybrid scheme” has been proposed in the PCT application for patent for invention with the application number PCT/CN2008/001443, and it may be viewed as the combinations of open-loop transmit diversity, Direction Of Arrival (DoA) based beam forming and the HARQ. Another typical example is to implement beam forming operation based on GoB (Grid of Beams) for SC MBMS as proposed by the PCT application for patent for invention with the application number PCT/CN 2008/001441.
Compared with two kinds of existing schemes for SC MBMS, that is, open-loop transmit diversity and max-min beam forming, although obvious performance improvement has been obtained by those enhanced algorithms in the applications for patent for invention with the application numbers PCT/CN2008/001441 and PCT/CN2008/001443, the resulted performance improvement is still limited mainly because all of MBMS users share the same physical time-frequency resources so as to cause that any operation performed on the physical resource is synchronously acting on all of MBMS receiving users. For the reason of random character of users' distribution, when technology such as max-min beam forming is used, while the main beam aims at the worst user, a plurality of other users may probably be located at side lobe of the main beam. This point may be further explained in FIGS. 1 and 2. FIG. 1 shows a schematic diagram of single cell MBMS transmission model. All of interested MBMS users receive MBMS service on the shared physical time-frequency resource as shown in FIG. 1, thus any operation has effect on all MBMS users. The core of two enhanced algorithms in the applications for invention with the application numbers PCT/CN2008/001441 and PCT/CN2008/001443 is the smart usage of beam forming. The normalized average beam patterns under 4 and 8 transmit antennas at eNB are shown in FIG. 2. It is clear from FIG. 2 that the main beam pattern becomes narrower with the increase of antenna numbers. Whenever the above narrow beam is used to aim at some users or user group, other users or user group may be located at side lobe of the main beam so as to incur performance loss due to the scattering of users' distribution within cell, namely random character.
As a result from above analysis, the performance improvement is still limited and more advanced technology is necessary to be investigated.